In a typical electrophotographic image forming device, such as copy machine and printer, now available in the market, a developer, a photosensitive member, or both are constructed in or as a disposable unit. Such unit, if it has reached a predetermined service life (e.g., it has been used for a predetermined service time or a predetermined number of printing or copying processes), should be replaced by a new one of the same type. For this purpose, the image forming apparatus needs not only a function for determining how long its has been used but also a function for determining whether a new unit is mounted in image forming device. Another function is needed for determining whether the unit is properly mounted in the image forming device.
To meet such requirements, U.S. Pat. No. 5,021,828 discloses a unit detector which includes not only a device for detecting whether the disposable unit mounted in the image apparatus is new but also a device for detecting whether the unit is properly mounted in the image forming apparatus.
The unit detector, however, uses two independent devices for respective purposes. This requires a number of components and then a balky space for accommodating such two devices.